"The smartphone is effectively becoming a scientific instrument," says Frank Moss of the MIT Media Lab. With modern high-resolution screens and powerful computing ability, the smartphone can perform tests that previously required a doctor's visit.

And more cheaply. Many of the medical apps on the market, or under development, require some sort of phone attachment that costs in the neighborhood of $100, says Eric Topol, a cardiologist at Scripps Clinic in La Jolla.

Plus, apps are fun and easy to use, Topol says. He's never had much luck persuading patients to take their blood pressure at home and send him the data. But people who keep phone-compatible blood pressure cuffs in the same place they charge the phone find it easy to slide on the cuff and hit start. And because the apps graph the information, patients often catch problems before Topol has a chance to check their blood pressure. Now they call him and suggest treatment changes.

Keep in mind there's a potential risk in trusting health decisions to an electronic gadget. "You want good information about the quality of these things," says Stephen Downs, chief technology and information officer at the Robert Wood Johnson Foundation in Princeton, N.J. He recommends checking an app's user feedback and the reputation of its developer. And don't rely on an app alone, Downs says, for important medical decisions.

Here are five medical tasks your phone can do:

Take your blood pressure

It looks much like your doctor's blood pressure cuff, with the addition of a connector to plug into your phone. The Withings monitor ($129), which you slide onto your upper arm, syncs up with a free app for the iPhone, iPad or iPod Touch. It saves information on your blood pressure and heart rate that you can email to your doctor or have automatically added to certain electronic health records. Another company, iHealth, will soon release a wireless wrist cuff for Apple devices.

Check your glucose

The iBGStar blood glucose monitoring system ($74.99 at Walgreens) attaches discreetly to the bottom of your iPhone or iPod Touch, so you can carry it anywhere. Just slide a blood test strip into the device to take a reading. On the free app, you can label readings with tags like "pre-breakfast," "post-dinner" or "night," and it will alert you when your blood sugar gets too high or low. You can also email the information to family members or your doctor.

Get your glasses prescription

Researchers at the MIT Media Lab are working on a plastic eyepiece — expected to cost a dollar or so — that snaps onto a mobile phone. The NETRA app asks you to peer through the eyepiece as you align spots or lines on a screen, and then calculates your prescription based on how much you had to move the spots to bring them together.

Monitor your heart rate

The new Cardiio app ($4.99) takes your heart rate with the iPhone or iPad camera by measuring how light reflects off your face as blood flows through your skin. And one day you may get a more detailed heart scan from your phone. AliveCor, which is under development, snaps onto the back of the phone like a case. When held to your chest, it conducts an electrocardiogram test of heart rhythm. A person worried about palpitations or dizziness could send a heart reading to a doctor and perhaps avoid an unnecessary emergency room visit, Topol says.

Diagnose an ear infection

The iPhone camera also works with CellScope's clip-on ear scope, or otoscope, currently undergoing testing and not yet for sale. The company hopes it will cut down on doctor visits for kids with recurring ear infections. With this app, parents could snap a picture of the ear's insides and send it to their doctor. CellScope is also working on a dermascope, which magnifies the skin to analyze rashes or moles.